At the time of its release, the manner on how it they’d marketed that was confusing as they showcased the Wii U gamepad as if its a portable console (its not) since its rather the main controller for the Wii U console. I was in high school when the console was first revealed, my reaction to that was “WTF?” as I thought the gamepad was portable, as the ads often flex that it can be (or the way they showed it came off as that).
In addition to what others have said, a large part of the problem here was that the marketing heavily focused on the GamePad rather than showing the whole console. So it was an understandable mistake to think the Wii U was a controller. That’s not a problem any of those other systems had, they were more clearly advertised as consoles.